Patients with stable coronary artery disease may experience episodes of myocardial ischemia during daily life which are often symptomatically silent. The presence of these episodes during ambulatory ECG monitoring is believed to provide prognostic information independent of that provided by exercise testing. However, the relationship between the findings of ambulatory ECG monitoring and the results of exercise testing is still undefined given the dissimilar results of previous reports. To further investigate this issue and, in particular, to establish the importance of the exercise protocol in determining the relationship between exercise testing and ambulatory ECG monitoring, we have studied 70 patients with stable coronary artery disease with 48-hour ambulatory ECG monitoring and treadmill exercise test using two different exercise protocols with slow (NIH Combined protocol) and brisk (Bruce protocol) increments in workload. All studies were carried out after withdrawal of medications. A close inverse correlation between ischemic threshold measured from exercise testing and the frequency and duration of ischemic episodes during ambulatory monitoring was observed using the NIH Combined protocol; however, a significantly weaker inverse correlation was found when using the Bruce protocol. The mean heart rate at the onset of ischemic episodes during monitoring did not differ significantly from the heart rate at the onset of ST segment depression during exercise testing with the NIH combined protocol, but it was significantly higher with the Bruce protocol. These findings indicate that a close relationship exists between ambulatory myocardial ischemia and the results of the exercise test; this relation is critically determined by the exercise protocol and is better observed with protocols that produce slow workload increments. These observations suggest that ambulatory ECG monitoring may not provide independent information in patients with stable ischemic heart disease and, therefore, does not need to be included in the routine evaluation of these patients.